1. Field of the Invention
This invention relates to an ink-jet head for recording letters or figures on the surface of a recording medium such as a sheet of paper by ejecting thereon small ink droplets and, more particularly, to a Fleming-type head which uses a magnetic and electric crossed-field applied to an ink flow path to motivate the generation of the ink droplets.
2. Description of the Related Art
A Fleming-type ink jet head is well known in which a magnetic and electric fields perpendicular to each other are applied transversely to an electroconductive ink flow passage, and in which a pulse like change is caused in one of the magnetic and electric fields to generate a driving force to eject ink through a nozzle. This type of head applies the driving force in only one direction and therefore has an improved efficiency and is capable of limiting the deterioration in the frequency response in comparison with other types of ink-jet heads (e.g., one based on pressurizing ink by utilizing deformation of a piezoelectric element caused by application of a voltage and a bubble jet type based on pressurizing by locally heating ink to form bubbles).
The Fleming-type ink jet head, however, entails the following problems to be solved.
To maintain the same printing quality as typewriters and dot printers presently put to practical use, it is necessary to arrange 24 or more nozzles of the above-noted ink jet head on a straight line at pitches of, for example, 0.14 mm in the ink-jet head. For this arrangement and for the purpose of ensuring high printing quality and reducing the manufacturing cost, it is important to devise a structural design enabling high-density integration of the nozzles, ink channels and electrodes adjacent to the channels.
2 In the ink-jet head, a surplus part of the ink to which the force of inertia is applied by an electromagnetic force such as that mentioned above, accumulates at the nozzle end. FIG. 1 schematically shows ink 1 forming a droplet and ink 2 (trickle of ink) accumulating and remaining at the nozzle end. In FIG. 1, reference numerals 3 and 4 designate the ink-jet head and the ink channel, respectively. Some part of ink 2 joins a droplet next ejected through the same nozzle. Consequently, if there is a trickle like ink 2, droplets of ink 1 cannot be formed uniformly in size, resulting in a deterioration in printing quality. To cope with this problem, a wiper for removing such a trickle of ink may be provided. However, the structure of the ink-jet head or of a printer in which the ink-jet head is used is thereby made complicated and the problem of a reduction in the printing speed due to the existence of the step of removing ink with the wiper is particularly serious.
3 Ordinarily, a common magnet is provided in association with ink channels to apply a magnetic field to the ink channels. The intensity of the magnetic field is smaller the channel is remoter from magnetic poles and closer to the center of the head because of divergence of magnetic flux, and the magnetic shielding effect of the electrodes, and the ink ejecting force is correspondingly reduced. Therefore droplets of ink 1 ejected through the nozzles are not uniform, resulting in a deterioration in printing quality
4 Inequality of electric currents flowing across the ink channels results in non-uniformity of the size of droplets of ink 1 and, hence, a deterioration in printing quality. Such inequality of electric currents is due to a change in the conductivity of ink, non-uniformity of the areas of the electrode surfaces facing each other, inequality of the resistances of current supply circuits including the electrodes, and other factors.
The change in the conductivity of ink also relates to variations in ejection frequency with respect to nozzles. That is, in a nozzle of a low ejection frequency, the viscosity of and the resistance of ink are increased as the solvent evaporates during staying in the ink channels. The size of ejected ink droplets is thereby changed.
To cope with this problem, a method of removing residual ink in each channel before the printing operation to supply ink at a constant density has been proposed. However, if this method is used, the structure of the ink-jet head or the printer using the ink-jet head is complicated and the consumption of ink is increased.